Some biologically active compounds contain cyclic peptide moieties and the study of peptides so constrained are also of importance in the study of their linear counterparts, the proteins. They serve as excellent model compounds in the study of protein conformations under controlled and stringent conditions. Cyclic peptides are also interesting in that they occur in different ring sizes and also contain non-protein amino acids like N-methyl and D-amino acids. The aim of this project is to investigate by x-ray crystallographic techniques the detailed structure and conformation of a few selected cyclic peptides.